Every gravity water filter gets a score from 0 to 100% built from six factors, which maps onto the 5-point rating shown on the site (a 0% score would be 1.5 stars; a perfect score, 5). Each factor is scored against fixed reference points, not against the other products in the list, so adding a new model never moves another model’s rating. Filters are grouped by tank capacity (up to 2.25 gal, 2.5 to 3.5 gal, and 4.5 gal and up) and use the same scale in every group.
| Factor | Measures | Weight |
|---|---|---|
| Value | price per gallon of tank capacity | 25% |
| Running cost | replacement element cost per gallon filtered | 15% |
| Capacity | tank size in gallons | 15% |
| Certification | NSF/ANSI standards the maker states are certified, with testing-only claims at the floor | 20% |
| Element life | rated gallons per element | 15% |
| Warranty | warranty length | 10% |
Where the numbers come from
Capacity, element type and life, certifications, flow and warranty come from the maker’s own pages and the maker’s copy on its Amazon listing. Berkey’s flow, size and warranty figures come from berkeywater.com. Prices are checked at Amazon, and the lowest price at a store the page links to is used. A figure the maker does not publish is left blank.
The factors
Value is the price divided by the tank capacity, in dollars per gallon, on a log scale. $250 per gallon scores zero and $50 per gallon full marks.
Running cost is the price of one replacement element divided by its rated gallons, on a log scale: $0.15 per gallon scores zero and $0.005 per gallon full marks. It is scored only where the maker publishes a gallon rating and an element price is known for that element. Several makers give element life in months only; those score as unknown.
Capacity is the tank size: 1.5 gal scores zero and 6 gal full marks.
Certification counts only standards the maker says it holds. NSF/ANSI 42 (aesthetic effects such as chlorine) and NSF/ANSI 53 (health effects such as lead) are worth 30 points each, NSF/ANSI P473 (PFAS) 25, NSF/ANSI 372 (lead-free materials) and 401 10 each, capped at 100. A maker that says only that its elements were tested, or meets some other standard, gets 10 points. A maker that names neither scores zero. Where a listing says IAPMO certified without naming the standard, only the standards it names are counted.
Element life is the maker’s rated gallons per element, on a log scale: 200 gal scores zero and 3,000 gal full marks. Makers who state only months are scored as unknown.
Warranty is the maker’s warranty on the system, not the elements: 6 months scores zero and 5 years full marks.
What isn’t scored
Flow rate, fluoride filters, height, weight, material and release date are listed but not scored. Flow rates are given with different numbers of elements by different makers, so they do not compare directly.
Missing data
If a figure is not published, that factor gets the neutral middle score (50) and the product page marks it as estimated. The exceptions are certification, where no claim at all scores zero, and warranty, where an unpublished warranty scores zero.
Ties and discontinued models
Products are ranked by score within each group. Discontinued models stay listed but rank below every current model and can be hidden with the “Hide discontinued” filter.